1. Field of the Invention
The present invention relates to a vessel for storing liquid fuel. In particular, the present invention relates to a liquid-fuel storage vessel suitably usable in a vapor jet system, such as a gas-liquid equilibrium thruster for use in a small-spacecraft propulsion system, and a vapor jet system using the vessel.
2. Description of the Background Art
A representative example of a conventional propulsion system for spacecrafts, such as satellites, includes a cold gas thruster, a hot gas thruster and an ion engine. The cold gas thruster includes one type as described, for example, in Tables 17-4 and 17-5 on pp 692-693 of “Space Mission Analysis and Design”, Space Technology Library, 1999. This propulsion system is designed based on the use of high-pressure nitrogen gas, and therefore requires a tank and a pipeline with high resistance to pressure, a pressure regulator and others. Thus, mounting of this propulsion system on a spacecraft leads to an increase in weight of the spacecraft. Further, in this system, fuel is loaded in a gas state. Thus, an energy density is relatively low, and a thrust to be obtained by this propulsion system will be gradually reduced due to pressure drop occurring along with a continuous jetting operation.
The hot gas thruster includes one type as described, for example, in “Liquid Propulsion Systems” on pp 24-26 of “Advanced Space Propulsion Systems”, Springer, 2003. This propulsion system requires a combustor, which leads to a complexity in configuration and an increase in weight of the system. Further, in this system, toxic hydrazine is predominantly used as fuel, which goes against advantage in handleability of a small spacecraft. The ion engine includes one type as described, for example, in “Ion Thruster” on pp 80-84 of “Advanced Space Propulsion Systems”, Springer, 2003. This propulsion system is required to ionize noble gas, such as xenon, which leads to extremely large power consumption. Further, this system is adapted to jet out extremely-light electrically-charged atoms at an ultra-high speed. Thus, a thrust to be obtained is considerably low, although a specific impulse is considerably high. Moreover, such a jet system cannot be made up without a complicated heavy mechanism.
As above, in case of applying the above conventional spacecraft propulsion systems to small-satellite attitude control or orbital maneuver, there exist problems, such as on-board space and weight, and complexity in configuration.
As a new technology concerning propulsion systems, a thruster using MEMS (abbreviation for Micro Electro Mechanical System: it is also referred to as “micromachine”) technologies is being researched and developed.
The thruster using MEMS technologies includes one type as described, for example, in “Micropropulsion” on pp 99-105 of “Advanced Space Propulsion Systems”, Springer, 2003. This system is an electromechanical type comprising micro-components fabricated by semiconductor technologies. However, as for space thrusters, this system is in a stage of research and development, but it has not been applied to an actual satellite. Further, this thruster has an extremely low thrust level. Thus, even if this thruster is mounted onto a satellite, it cannot contribute to rapid attitude control or orbital maneuver.
As compared with the propulsion systems having the above problems, a gas-liquid equilibrium thruster has great potential as a small-satellite propulsion system. The gas-liquid equilibrium thruster is adapted to jet out only vapor by means of a vapor pressure of a liquid used as fuel. This provides an advantage of being able to eliminate the need for a gas reservoir and a combustor so as to achieve a reduction in weight and a simplification in configuration of the system. A propulsion system using the principle of gas-liquid equilibrium includes a thruster developed for the small satellite SNAP-1, as described, for example, in D. Gibbon, J. Ward, N. Kay, “The Design, development and Testing of a Propulsion System for the SNAP-1 Nanosatellite”, Proceedings of the 14th Annual AIAA/USA Conference on Small Satellites, Logan, Utah, Aug. 21-24, 2000.
The thruster for SNAP-1 doesn't fully utilize the benefit of the gas-liquid equilibrium thruster and has a problem in terms of storage and vaporization of liquid fuel. Specifically, in case of employing a technique of loading liquid fuel in a pipeline, there are restrictions on fuel loading capacity. Further, a temperature control mechanism is simply provided adjacent to a fuel jetting nozzle in order to prevent a mist ejection which is an undesirable phenomenon that liquid fuel is jetted out directly without being vaporized. Thus, the mist ejection cannot be adequately avoided to maintain a stable thrust level. Moreover, during spacecraft attitude control, the loaded fuel in the pipe line is freely moved, which is likely to produce vibration causing a sloshing phenomenon.
[Non-Patent Document 1] “Space Mission Analysis and Design”, Space Technology Library, 1999, pp 692-693
[Non-Patent Document 2] “Advanced Space Propulsion Systems”, Springer, 2003, “Liquid Propulsion Systems”, pp 24-26
[Non-Patent Document 3] “Advanced Space Propulsion Systems”, Springer, 2003, “Ion Thruster”, pp 80-84
[Non-Patent Document 4] “Advanced Space Propulsion Systems”, Springer, 2003, “Micropropulsion”, pp 99-105
[Non-Patent Document 5] D. Gibbon, J. Ward, N. Kay, “The Design, development and Testing of a Propulsion System for the SNAP-1 Nanosatellite”, Proceedings of the 14th Annual AIAA/USA Conference on Small Satellites, Logan, Utah, Aug. 21-24, 2000
As a solution for the above problems in the thruster of SNAP-1, it is contemplated to employ a technique of storing liquid fuel in a widely-used hollow tank, instead of loading liquid fuel in a pipeline.
However, according to inventors' knowledge, in case where a hollow tank is used in a gas-liquid equilibrium thruster to store liquid fuel, due to occurrence of the incidental phenomenon that liquid fuel stored in a tank is jetted out directly without being vaporized, a specific impulse is significantly lowered to cause deterioration in fuel efficiency.
Further, in the case of using a hollow tank, when an internal pressure of the tank becomes lower along with a jetting operation, local bumping easily occurs due to a small ratio of a surface area to a volume of the liquid fuel, which cause instability in thrust level.
Moreover, when the internal pressure of the tank becomes lower along with a jetting operation, the liquid fuel is vaporized, and an internal temperature of the tank is reduced due to a latent heat of vaporization. This precludes the internal pressure of the tank from returning to a value before the jetting operation, and thereby the thrust level is lowered. Thus, it is necessary to heat the tank by a heater in order to compensate for the reduction in the internal temperature. However, if the tank is simply heated from an outside thereof by the heater, heat from the heater is hardly transferred to the liquid fuel in the tank in a uniform manner.
In case where a hollow tank is used in a gas-liquid equilibrium thruster mounted on a spacecraft, such as a satellite, to store liquid fuel, the liquid fuel in the tank can be freely moved within the tank during spacecraft attitude control, to cause a sloshing phenomenon having adverse effects on the spacecraft attitude control.